Chemical vapor deposition (CVD) is used to apply single or multiple layers of coatings to substrate surfaces. Typically, the thickness of each coating layer may be on the order of a fraction of a micron to around twenty microns. In CVD, a gas or gases containing the atoms that will make up the coating are reduced or decomposed on or very near to a substrate surface at high temperatures so that a coating of a desired composition is deposited onto the substrate. The deposition can be a metal, a semiconductor, an alloy or a refractory compound.
MT CVD differs from CVD in that the reaction temperatures used for MT CVD are significantly lower than those used for CVD. Often the temperature difference is on the order of hundreds of degrees. One way of achieving the reduced temperature of MT CVD is by the use of a reactant gas or gases that are capable of reacting to form the desired coating at the lower temperatures.
Titanium carbonitride (TiCN) coatings can be deposited by either CVD or MT CVD. The deposition of a TiCN coating by CVD and by MT CVD has been found to be useful for imparting a wear resistant, hard coating on substrates. An example of a CVD process for depositing TiCN coatings is one which uses a deposition process gas containing methane (CH.sub.4) as a reactant along with titanium tetrachloride (TiCl.sub.4) and molecular hydrogen (H.sub.2) to form a TiCN coating at a reaction temperature of about 1000.degree. C. If the reactant gas CH.sub.4 in this deposition process gas is replaced by acetonitrile (CH.sub.3 CN gas), a MT CVD process is obtained by which a TiCN coating can be deposited on a substrate in the reaction temperature range of 700 to 900.degree. C.
In MT CVD processes which use CH.sub.3 CN in the deposition process gas along with TiCl.sub.4 and H.sub.2, a layer of TiCN is formed by a reaction that is represented by the following formula: EQU TiCl.sub.4 +CH.sub.3 CN+5/2 H.sub.2 .fwdarw.TiCN+4HCl+CH.sub.4.
The use of MT CVD processes employing CH.sub.3 CN as part of the deposition process gas for coating substrates with one or more layers of TiCN is well known in the art. For example, Bitzer et al., U.S. Pat. No. 4,196,233, describes processes for coating inorganic substrates with, among other things, carbonitrides, and discloses the use of a MT CVD coating process employing CH.sub.3 CN. A. T. Santhanam and D. T. Quinto, "Surface Engineering of Carbide, Cermet, and Ceramic Cutting Tools," ASM Handbook, Vol. 5, Surface Engineering (1994), pages 900-908, relate that in the mid-1980s a MT CVD process for depositing TiCN on cemented carbides was commercialized. They describe that process as using a deposition process gas containing a mixture of TiCl.sub.4, H.sub.2, and an organic carbon-nitrogen compound such as CH.sub.3 CN and a reaction temperature of 700 to 900.degree. C. to produce faster deposition rates at lower temperatures than conventional CVD processes. They relate that the process had the advantage of producing fewer thermally induced tensile cracks than the higher temperature conventional CVD processes.
Over the years, refinements to MT CVD processes employing CH.sub.3 CN have been made. For example, Odani et al, U.S. Pat. No. 5,436,071 describes the application of a TiCN coating on cermets using a reaction gas composed of 0.1.about.1% of CH.sub.3 CN, 1.about.5% of TiCl.sub.4, 0.about.25% of molecular nitrogen (N.sub.2) with the remaining portion being composed of H.sub.2, under a reaction temperature of 800.about.900.degree. C. and a reaction pressure of 30.about.200 Torr.
Although the prior art MT CVD processes have been commercially useful, they have some disadvantages. One disadvantage which is of particular importance commercially is that the rate of deposition of TiCN, although faster than that of some CVD processes, is still somewhat slow. The slow deposition rate contributes to the long batch cycle times needed to coat substrates and thereby adversely affects production rates.
Another disadvantage encountered with prior art MT CVD processes is that they often produce a variation in coating thicknesses throughout the MT CVD reaction chamber. Due to this problem, substrates located near the entrance of the deposition process gas often have significantly thicker coatings than those located farther away from the deposition process gas entrance.